Southern California -- this just in

Officials warn of freezing conditions on Thanksgiving morning

November 24, 2010 | 1:32
pm

A freeze advisory was issued Wednesday for parts of Southern California, where overnight temperatures are expected to dive in valley, mountain and high desert areas.

The warning extends through 9 a.m. on Thanksgiving Day. Overnight, officials expect temperatures in the 20s and 30s in the San Fernando and San Gabriel valleys and 16 to 22 degrees in the Antelope Valley. Temperatures in the L.A. basin will be slightly high, at about 40 degrees, forecasters said.

Courtesy of the Gulf of Alaska, the jet stream has been swooping down and bringing storms and cold air to the Pacific Northwest, down through Northern California to the Southland. It's been cold, and it's expected to stay that way through the holiday weekend. There's a slight chance of rain Saturday or Sunday.

Monday's high temperature was nine degrees below normal, and Tuesday, when the temperature dipped to 61 degrees, it was six degrees below normal, said Bill Patzert, a climatologist for the Jet Propulsion Laboratory in La Cañada-Flintridge.

That's not exactly Donner party weather, or anywhere as frigid as it is in much of the rest of the country, but it still qualifies as cold for sun-spoiled Southern Californians. "It’s going to be cold and windy pretty much. That's the story of Thanksgiving," said Bonnie Bartling of the National Weather Service in Oxnard. "We're having a cold Santa Ana event."

Though it's been cooler than normal recently, the Los Angeles area hasn't exactly had many normal days of late, weather experts say. Downtown L.A. recorded temperatures in the mid- to high 90s early in the month, before temperatures quickly dipped into the 70s. Then the temperatures toggled between the 60s and a few 70s before descending back into the 60s. Mt. Wilson is expected to hover in the mid-40s, and desert communities such as Lancaster are expected to see temperatures in the high 40s, with nighttime temperatures expected to go down below 20 degrees, Bartling said.

The valleys, and much of the rest of L.A., including the beach areas, are expected to remain in the 60s through the weekend. Patzert said that despite light rains, L.A. has recorded lower than normal rainfall, with only about 1.35 inches of rain falling in downtown.

That, along with the cooler temperatures, is consistent with cooling temperatures in the equatorial Pacific strengthening into a relatively vigorous La Niña. Unlike El Niños, which often inspire warmer and wetter than average conditions in Southern California, La Niñas are more reliable omens of dry and cold temperatures, Patzert said.

Since 1949, there have been 22 La Niñas, and during 18 of them, the region saw below normal rainfall, he said. But an "amped up" jet stream from up north could boost the chances of rain. Still, the only sure thing for Thanksgiving weekend, Patzert and Bartling said, seems to be cold.

"This is the kind of thing you usually get in December and January, where you really get a cold snap," Patzert said.